Various diagnostic procedures require IOP measurements, most widely used for diagnosing glaucoma condition, i.e., increase in IOP that can cause optic nerve degeneration which results in loss of vision. Conventional IOP measurement systems are based on the principles of a mechanical tonometer, namely a direct pressure measurement. Such tonometers are of contact and non-contact types. The contact type systems utilize a measuring probe directly pressed against a patient's eyeball, and measurement of a pressure corresponding to a given degree of deformation; in the non-contact measurement systems, a stream of air is blown onto the cornea surface, and a deformation of the cornea caused by this stream is measured.
JP 5056932 discloses a tonometer aimed at enabling a patient to measure his own intra-ocular pressure with ease at home, etc. The patient holds an outside cylinder of the tonometer device and presses a contact member to the eye to be examined in such a manner that this member comes into contact with the eye by crossing the eyelid. A spring which is contracted by a pressing force is used, and is associated with a sensor, such that light emitted from a light source is made incident on the sensor via a mirror, when the spring attains a prescribed value. This light acts as a signal to cause electromagnetic driving parts of the device to drive a press-fitting member, mounted coaxial with the contact member, toward the eye to be examined. The moving quantity of the press-fitting member of this time is detected by another light source and an array sensor, and the intra-ocular pressure is measured from the driving force and the moving quantity of the press-fitting member.
U.S. Pat. No. 5,349,955 discloses a tonometer adapted to measure intra-ocular pressure by applying pressure onto an eye being examined via its eyelid. The tonometer has a pressure means that presses against the eyelid of a subject eye to thereby apply pressure to the eye. The load applied to the pressure means is detected by a load sensor and used to calculate intra-ocular pressure of the eye. The tonometer is structurally simple and compact, operationally safe, and does not cause the patient to feel fear and discomfort.
U.S. Pat. No. 6,440,070 discloses an intra-ocular pressure measurement apparatus including a housing and a processor. The housing has a protuberance with a flat surface, and the processor includes a distance-measuring unit and a force-measuring unit. Intra-ocular pressure is calculated based on force/distance relationships, where the distance includes a measurement to an internal element of the eye. The processor calculates intra-ocular pressure by determining a zero displacement pressure.
The most widely used tonometers are based on the principles of the Goldmann Applanation Tonometer of direct pressure measurement following applanation of a cornea. According to the “Gold Standard”, a flattened diameter optimal to this technique is chosen to define a flat contact surface such that no component of corneal tension is perpendicular to the cornea/tonometer interface.